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When inhaling, humans take in approximately 21 percent oxygen, 0.04 percent carbon dioxide and 79 percent nitrogen. On exhalation, humans give off approximately 16 percent oxygen, 4 percent carbon dioxide and 79 percent nitrogen, according to the BBC; only the amount of nitrogen remains constant in the exchange. Exhaled air also contains water vapor. Red blood cells travel by vein to the lung; they give of carbon dioxide and take in oxygen.